The disciples had seen the pillar of cloud on the mountain ((Acts 1:9)) , and now they saw the other manifestation of God: the pillar of flame ((Genesis 15:17, Exodus 3:2, 13:21, 19:18, 24:17, 2 Samuel 22:13, 2 Kings 6:17, Psalms 50:3, Ezekiel 1:27)). The roaring fire rushed into the upper room, illuminating everything inside. The startled disciples watched as it spread out, marking each of them with a flame ((Acts 2:3)). As the flames over their heads faded, a new flame ignited in their spirits.
They felt a connection with something, someone, more wise, more powerful, more holy than themselves. Though he could easily have consumed them, instead he prompted gently. Down below, in the temple, a multitude of their countrymen had come to offer the first fruits of their harvest. Now they saw it clearly: They were the first fruits of a different kind of harvest, and it was time to offer themselves in the temple.
As one, they rushed from their room and into the waiting crowd, eager to tell those still in darkness about what they had seen: eager to spread the fire of heaven ((Acts 2:4)).